The invention relates to a catheter system adapted for pressure sensing.
Such a catheter system is used for percutane transluminal angioplastic. During the treatment of stenosis of blood vessels is it important to be able to measure the blood pressure instantaneously. By measuring the amount of blood flow concurrently with measuring the pressure, one obtains information regarding the degree of obstruction, provided that pressure sensing is accurate. Prior art pressure sensing systems usually are not. Known is for instance a system for measuring pressure at an opening at the end of a catheter, or a measuring system in which the pressure is measured unilaterally through a small hole in the side of the catheter. The fact that the pressure measurement at an opening at the end of the catheter, the normal of the opening being substantially parallel to the direction of the blood flow, is inaccurate follows from the law of Bernouilli. Unilateral pressure measurement through a small hole in the side of the catheter is problematic because the pressure measurement in that case is dependent on the accidental rotational position of the catheter about its longitudinal axis. The pressure distribution around the catheter is hardly ever homogeneous, since the catheter merely always lies against the wall of the blood vessel in which it is present.
It is a first goal of the invention to provide a catheter system for being able to perform an accurate pressure measurement.
It is a second goal of the invention to provide a catheter system that may be used in so-called interventional MRI. Interventional MRI is a treatment technique in which the patient undergoes treatment when being in a magnet scanner (MRI). By providing a catheter system that may be used during this treatment, important advantages for treatment can be reached since the surgeon may obtain current information.
It is a third goal of the invention to provide a catheter system with which also rapid pressure changes can be recorded well in time, without pressure pulses, such as those originating from the heart beat, being smoothed.
These goals are achieved by the catheter system according to the invention being characterized in that it comprises a transmitter/receiver for polarized light and an optical fiber to be connected to said transmitter/receiver, said fiber at its distal end being provided with a cylindrical body of transparent plastic material, and said body having a hollow central inner lumen provided around its longitudinal axis, and in that said cylindrical body is provided with a mirror at its side facing away from said fiber. The invention is also embodied in a separate catheter which may or may not be used as a guiding thread in said catheter system, and which according to the invention at its distal end is provided with a cylindrical body of transparent plastic material, said body having a hollow central inner lumen provided around its longitudinal axis, and said cylindrical body being provided with a mirror at the side facing away from said fiber. Such a catheter lacks ferromagnetic materials and long electrical conductors which would prevent use of the catheter in interventional MRI. Preferably A-PET is being used as the plastic material.
The invention is based on the understanding that the cylindrical body of transparent plastic material which is provided at the distal end of the optical fiber has birefringent characteristics which are dependent on the pressure present on the outside of the body, i.e. the propagation speed of the laser light travelling through the plastic body is dependent on the direction of polarization of said light. When a pressure is present on the outside of the cylindrical body, part of the light of a first polarization state converts to a second polarization state, said second state being perpendicular to the original polarization state. The degree of the conversion is dependent on the pressure exerted on the cylindrical body. Measuring the amount of light converted in polarization direction yields an indirect measure of the pressure. By constructing the plastic body cylindrical one obtains in addition a symmetrical pressure sensor which is sensitive on all sides, the rotational position of the catheter not being important.
It is desirable that a lens is provided in between said distal end of said fiber and said cylindrical body. The lens is being employed to transform the divergent light leaving the fiber into a collimated beam of polarized laser light.
Furthermore, it is desirable that the inner lumen at its side facing away from said fiber is provided with a black body. This has the effect that the light propagating through the inner lumen is extinguished, and guarantees that the light reflected by the mirror passes the plastic body twice.
For a good operation of the catheter system according to the invention, it is desirable that said transmitter/receiver comprises a laser source and a first polarization filter arranged in between said laser source and a beam splitter having a fixed polarization. The first polarization filter is being employed to condition the light originating from the laser source for the pressure measurement.
It is furthermore desirable that a second polarization filter and a detector positioned behind said second polarization filter are connected to said beam splitter.
Measuring the pressure with the catheter system according to the invention is easily realized when said first and second polarization filters polarize the laser light mutually orthogonal.